There are numerous applications where it is desired to convert DC power into AC power including one or more phase voltages and a neutral voltage. For example, in a variable speed, constant frequency (VSCF) power generating system, variable frequency AC power is produced by a brushless, synchronous generator driven by a variable speed prime mover. The variable frequency AC power is converted into DC power on a DC link by a rectifier bridge and the DC power is converted into polyphase constant frequency AC power by a polyphase inverter. The inverter may be operated by a full bridge control which develops a fundamental AC component of relatively large amplitude and harmonics, in particular triplen harmonics, generally having lesser amplitudes. A neutral voltage is typically formed at a neutral terminal of a neutral forming transformer (NFT) coupled to the inverter output. The neutral voltage, however, is not fixed relative to the DC voltage on the DC link. Unbalanced currents in such a system resulting from unbalanced loads flow from the neutral terminal through the windings of the NFT and the inverter switches back to the generator. Hence, unbalanced loads are a determining factor in sizing system components, in particular the NFT, which adds significant size and weight to the inverter. This, in turn, may render the VSCF system unsuitable for certain applications, such as in aircraft or other airborne equipment. Instead of the full bridge inverter control, the inverter may be operated by a half bridge control which develops a fundamental component at a relatively lower amplitude and hence the DC voltage on the DC link must be higher for a given inverter output voltage magnitude as compared with an inverter operated by a full bridge control. However, a neutral voltage can be formed on the input side of the inverter so that the neutral is fixed relative to the voltage on the DC link. This, in conjunction with the fact that all of the inverter phases are individually controllable, facilitates the conduction of neutral currents and thus the handling of unbalanced loads. However, the half bridge control causes the inverter to develop harmonics in the inverter output which are not as easily filtered as the harmonics produced using the full bridge control.
Turnbull, U.S. Pat. No. 3,775,663, discloses an inverter having an electronically controlled neutral forming circuit. The circuit includes a pair of switches coupled across a DC link and a control for operating the switches to maintain a junction between the switches at a neutral voltage.